1. Field of the Invention
The present invention relates generally to a rotary drilling arrangement for mitigating pressure-differential sticking of a drill string in a wellbore. More particularly, the subject invention concerns a method and apparatus for drilling deviated wellbores, such as in extended reach drilling, which are particularly designed to reduce the chance of pressure-differential sticking of the drill string in the wellbore.
Extended Reach Drilling is concerned with rotary drilling procedures to drill, log and complete wellbores at significantly greater inclinations and/or over horizontal distances substantially greater than currently being achieved by conventional directional drilling practices. The success of extended reach drilling should benefit mainly offshore drilling projects as platform costs are a major factor in most offshore production operations. Extended reach drilling offers significant potential for (1) developing offshore reservoirs not otherwise considered to be economical, (2) tapping sections of reservoirs presently considered beyond economical or technological reach, (3) accelerating production by longer intervals in the producing formation due to the high angle holes, (4) requiring fewer platforms to develop large reservoirs, (5) providing an alternative for some subsea completions, and (6) drilling under shipping fairways or to other areas presently unreachable.
A number of problems are presented by high angle extended reach directional drilling. In greater particularity, hole inclinations of 60.degree. or greater, combined with long sections of hole or complex wellbore profiles present significant problems which need to be overcome in extended reach drilling. The force of gravity, coefficients of friction, and mud particle settling are the major physical phenomena of concern.
As inclination increases, the available weight from gravity to move the pipe or wireline string down the hole decreases as the cosine of the inclination angle, and the weight lying against the low side of the hole increases as the sine of the inclination angle. The force resisting the movement of the drill string is the product of the apparent coefficient of friction and the sum of the forces pressing the string against the wall. At an apparent coefficient of friction of approximately 0.58 for a common water base mud, drill strings tend to slide into the hole at inclination angles up to approximately 60.degree. . At higher inclination angles, the drill strings will not lower from the force of gravity alone, and must be mechanically pushed or pulled, or alternatively the coefficients of friction can be reduced. Since logging wirelines cannot be pushed, conventional wireline logging is one of the first functions to encounter difficulties in this type of operation.
Hole cleaning also becomes more of a problem in high angle bore holes because particles need fall only a few inches to be out of the mud flow stream and to come to rest on the low side of the hole, usually in a flow-shaded area alongside the pipe. This problem is also encountered in substantially vertical wellbores but the problem is much worse in deviated wellbores. In deviated wellbores the drill string tends to lie on the lower side of the wellbore and drill cuttings tend to settle and accumulate along the lower side of the wellbore about the drill string. This condition of having drill cuttings lying along the lower side of the wellbore about the drill string along with the usual filter cake on the wellbore wall presents conditions susceptible for differential sticking of the drill pipe when a porous formation is penetrated that has internal pressures less than the pressures existing in the borehole.
This settling of cuttings is particularly significant in the near horizontal holes expected to be drilled in extended reach drilling. Present drill strings of drill pipe body, tool joints and drill collars are usually round and rotate concentrically about a common axis. If the pipe rotates concentrically around the same axis as the tool joints which are normally positioned against the solid wall and act as bearings for the rotating string, then a long "keyseat" is developed as the pipe is buried and beds itself into the cuttings and wall cake. A similar action of a drill string rotating about a concentric axis in a thick wall cake in a vertical hole could produce the same results. If differential pressure (borehole mud pressure less formation pore pressure) exists opposite a permeable zone in the formation, then conditions in both cases are set for the pipe to become differentially wall stuck. In both cases, the pipe is partially buried and bedded into a mass of solids, and can be hydraulically sealed to such an extent that there is a substantial pressure difference in the interface of the pipe and the wall and the space in the open borehole. This hydraulic seal provides an area on the pipe for the pressure differential to force the pipe hard against the wall. The frictional resistance to movement of the pipe against the wall causes the pipe to become immovable, and the pipe is in a state which is commonly referred to as differentially stuck.
2. Discussion of the Prior Art
Pressure-differential sticking of a drill pipe is also discussed in a paper entitled "Pressure-Differential Sticking of Drill Pipe and How It Can Be Avoided Or Relieved" by W. E. Helmick and A. J. Longley, presented at the Spring Meeting of the Pacific Coast District, Division of Production, Los Angeles, Cal., in May 1957. This paper states that the theory of pressure-differential sticking was first suggested when it was noted that spotting of oil would free pipe that had stuck while remaining motionless opposite a permeable bed. This was particularly noticeable in a field wherein a depleted zone at 4300 feet with a pressure gradient of 0.035 psi per foot was penetrated by directional holes with mud having hydrostatic gradients of 0.52 psi per foot. In view thereof, it was concluded that the drill collars lay against the filter cake on the low side of the hole, and that the pressure differential acted against the area of the pipe in contact with the isolated cake with sufficient force that a direct pull could not effect release. This paper notes that methods of effecting the release of such a pipe include the use of spotting oil to wet the pipe, thereby relieving the differential pressure, or the step of washing with water to lower the pressure differential by reducing the hydrostatic head. Field application of the principles found in a study discussed in this paper demonstrate that the best manner for dealing with differential sticking is to prevent it by the use of drill collar stabilizers or, more importantly, by intentionally shortening the intervals of time when pipe is at rest opposite permeable formations.
Fox U.S. Pat. No. 3,146,611 discloses tubular drill string members formed with grooves along continuous paths which are designed to reduce the area of periphery engagement with the wellbore and thereby lessen the likelihood of the members becoming stuck due to a pressure differential.
William Jr. U.S. Pat. No. 3,306,378 describes special drill collars used in a drill string for boring holes which are designed to maintain a stiff stem above the drill bit to counteract the tendency of the drill collars to flex and corkscrew and thus increase the drilling weight without causing a deviation of the bit. In this approach drill collars having an eccentric hole therethrough are connected with the drill pipe by means of tool joint connections on the ends thereof such that the drill collars gyrate in continuous contact with the borehole wall. Two or more collars are arranged symmetrically about the axis of rotation to maintain a uniformity of support on the wall of the borehole and also to provide the stiffness required to maintain linear alignment of the bit with respect to the axis of rotation.
Williams Jr. U.S. Pat. No. 3,382,938 describes another method for controlling deviation of a drill bit from its intended course by providing drill collars which carry a series of spaced-apart pads extending radially from one side of the collar and having faces in wiping contact with the wall of the borehole.
Dunn U.S. Pat. No. 2,841,366 discloses a method and apparatus for drilling wells which are concerned with controlling and stabilizing the drill collars and bit at the lower end of a drill string. The action of the drill collars and bit is controlled and stabilized by the provision of an eccentric weight. At a point where the drill collars tend to buckle and bend, a drill collar is provided that has generally aligned upper and lower coupling portions and an eccentric intermediate portion. The eccentric intermediate portion swings by action of centrifugal force in a circular path around the wellbore, and has a wiping engagement with the side of the wellbore which tends to smooth the wall thereof. As the eccentric portion revolves, the aligned portions are held concentric with the central axis of the wellbore and hold the drill bit vertically disposed such that the earth is penetrated in a manner to produce a straight, vertical bore.
Arnold U.S. Pat. No. 3,391,749 discusses a technique for preventing a well borehole from deviating from the vertical as it is being drilled by using a drill collar which is eccentrically weighted with respect to its axis of rotation.
Sanders U.S. Pat. No. 2,309,791 discloses a method and apparatus for cementing casing in a well wherein the casing is pushed away from the walls thereof. Stringers of mud which tend to remain in place as cement slurry flows upwardly around the casing and are broken up so that the casing is completely surrounded by cement. The casing is provided with eccentric enlargements. Either by orientation of such enlargements with respect to the casing or rotation of the casing, or by a combination of the two, the casing tends to be centered in the hole. These eccentric enlargements can be carried by or comprised of a coupling, shoe, float collar, or any fitting placed in the casing string. Rotation of the eccentric enlargements disturbs the flow of an ascending cement column, tending to force it around all of the sides of the casing.
Square and triangular drill collars have been used in many boreholes. However the purpose for their use was to attain stiffness of the bottom-hole assembly, not for preventing differential wall sticking. Spiral grooves have also been used for preventing differential wall sticking. However, spiral grooves are not similar to the out-of-round cross-sectional shape disclosed and taught herein.